(1) Field of the Invention
The present invention relates to a Schmitt trigger circuit which exhibits hysteresis in response to an input voltage.
(2) Description of the Prior Art
In a Schmitt trigger circuit, the threshold level is high with respect to the rise of an input voltage (from an L level to an H level) and is low with respect to the fall of the input voltage (from the H level to the L level). This hysteresis means a Schmitt trigger circuit has large noise margins and furthermore is free from the oscillation which plagues a transistor-transistor logic (TTL) circuit having a fixed threshold level. Therefore, a Schmitt trigger circuit is preferably used for an input buffer of a long bus line in which noise easily enters and causes distortion of the wave shapes of signals therein.
A conventional type of Schmitt trigger circuit used for input buffers has a serious problem in that the input current abruptly increases by a great deal when the input voltage falls.